1. Field of Invention
The present invention relates generally to high temperature polymers. It relates particularly to a semi-interpenetrating polymer network approach to the obtainment of more processable, tougher and more moisture resistant high temperature polymers. The systems are particularly adapted to use as moldings, adhesives and composite matrices.
2. Description of the Related Art
There is a continual search in the art for more processable and damage tolerant high temperature polymers for use as moldings, adhesives and composite matrices in aerospace and electronic technologies. Materials used in these environments must have a variety of desired properties including easy processing, good damage tolerance, a high glass transition temperature, good mechanical performance, capable of withstanding high temperature, low moisture absorption, and resistance to a variety of organic solvents. Although polymers exist that exhibit one or more of the above properties, these materials are generally deficient in at least one other desired property.
One example of such material is the thermoplastic polyimide, NR-150B2, which is commercially available from E.I. Dupont de Nemours and Company (Dupont). This material is well known for its good toughness and microcracking resistance. In addition, it has unusually high thermo-oxidative stability. Unfortunately, it is difficult to process and it requires processing temperatures as high as 400.degree. C.
Another example includes the commercially available Thermid.RTM. materials, which are commercially available from the National Starch and Chemical Corporation. These materials are acetylene-endcapped polyimides. They offer outstanding thermo-oxidative stability, exceptional dielectric properties and excellent resistance to humidity at elevated temperature. However, these materials are inherently brittle due to their highly crosslinked structures and are liable to microcrack in their composites when subjected to thermal cycling. Also, despite having the advantage of addition-curing, they are actually very difficult to process. This is primarily due to their very narrow processing window. Thermid.RTM. MC-600, for example, has a gel time of three minutes at 190.degree. C. (A. L. Landis and A. B. Naselow, NASA Conference Publication 2385 (1985)). The problem becomes exacerbated in composite fabrication, particularly in large and/or complex composite parts. Because of the processing difficulty, the composite property values for Thermid.RTM. MC-600 are lower than expected. The National Starch and Chemical Corporation product data sheet number 26283 reports the values of 195 and 148 ksi for the unidirectional flexural strengths tested at 25.degree. C. and 316.degree. C., respectively, and interlaminar shear strengths of 12 and 8 ksi at 25.degree. C. and 316.degree. C., respectively. The desired values are 250 and 150 ksi for the 25.degree. C. and 316.degree. C. flexural strengths and 14 and 8 ksi for the 25.degree. C. and 316.degree. C. interlaminar shear strengths.
This processing problem was well recognized in the early stages of the material's development. Several approaches have been attempted to improve the processability of Thermid.RTM. MC-600. The first approach was to incorporate difunctional or monofunctional acetylene-terminated reactive diluents into the material (A. L. Landis and A. B. Naselow NASA Conference Publication 2385 (1985)). This approach had limited success due to the lack of a common solubility between the preimidized oligomer and the diluent.
Grimes and Reinhad (U.S. Pat. No. 4,365,034) took another approach, recognizing that the processing problem was related to the fast cure rate of the acetylene-terminated material. They added a chemical inhibitor to retard the rate of cure so that the oligomer remains in the fluid state for an extended period of time thereby increasing the processing window. Some examples of this inhibitor include hydroquinone, maleic acid, glutaric acid, or bis(.beta.-naphthyl)-para-phenylene diamine. However, whether such an approach indeed facilitates the fabrication of high quality composite materials was not demonstrated.
To improve the resin flow, Landis and Naselow (NASA Conference Publication 2385 (1985)) developed an isoimide version of Thermid.RTM. MC-600, which is now known as Thermid.RTM. IP-600. Despite the markedly improved resin flow, the resulting composite showed relatively low levels of mechanical properties (unidirectional flexural strengths of 130 and 78 ksi at 25.degree. C. and 288.degree. C. and interlaminar shear strengths of 7 and 5 ksi at 25.degree. C. and 288.degree. C., respectively).
Recently, Landis and Lau (U.S. Pat. No. 4,996,101) extended the isoimide modification concept to the development of a semi-interpenetrating polymer network (semi-IPN). They prepared a sequential semi-2-IPN by combining a thermoplastic polyisoimide with a thermosetting imide or isoimide oligomer which contains an acetylene-terminated group. They assert that the isoimide modification can, by theory, improve the composite processing and thereby produce better quality composite materials than the present state-of-the-art materials. Unfortunately, they did not demonstrate the improved composite properties for these semi-2-IPNs. The absence of a showing of the composite mechanical properties makes the utility of this technology questionable. It is doubtful that the isoimide modification can, in practice, significantly improve the processability. The reason is as follows: the isoimide undergoes an isoimide-imide isomerization. This isomerization reaction induces a melt-flow transition which is responsible for the improved resin flow. However, the isomerization reaction takes place rapidly and occurs at a relatively low temperature. Thermid.RTM. IP-600, for example, shows a sharp melt-flow transition peak at 148.degree. C. in the Rheometrics.RTM. rheology-temperature curve. This is illustrated in FIG. 1. This transition is due to the isoimide-imide isomerization. This interpretation is supported by the appearance of another transition peak occurring at 188.degree. C. due to the melt-flow of the imide formed from the isoimide. Thermid.RTM. MC-600 has the same transition peak at 188.degree. C. The cure temperature for Thermid.RTM. based composites is usually 250.degree. C. At this critical cure temperature, Thermid.RTM. IP-600 has already gelled, the gel temperature being 220.degree. C. Thus, the enhanced flow resulting from the isoimide modification will not significantly affect the composite fabrication. This may explain why low values were obtained for the composite mechanical properties formed from Thermid.RTM. IP-600.
Egli and St. Clair (U.S. Pat. No. 4,695,610) have also prepared chemically compatible semi-2-IPNs from thermoplastic polyimide sulfones and thermosetting acetylene-endcapped polyimide sulfones. However, none of these prior art products have the desired combination of properties set forth herein above.
It is believed that the processing difficulty of the Thermid.RTM. materials is directly related to the fundamental nature of the curing chemistry. According to the proposed cure mechanism (Foldfarb, Lee, Arnold, and Helminiak, NASA Conference Publication 2385 (1985)) the curing of an acetylene-terminated oligomer proceeds stepwise and can be broadly divided into two distinct stages. The reaction sequence is shown by the following reaction scheme. ##STR1##
In stage one, the reaction site is an acetylene-terminated group, which is marked in the rectangular area at the top of the reaction equation. This reacting group is relatively sterically unhindered and is ready to react with another acetylene-terminated group of a different molecule. The addition reaction occurs very rapidly via a free radical mechanism. In a very short period of time, six to seven molecules are added to form a cluster, which has six to seven arms and a conjugated polyene moiety embedded in the center of the cluster. At this stage, the material is in the solid state. The reaction essentially stops until a higher curing temperature is applied.
The fast reaction rate of the stage one reaction is responsible for the narrow processing window of an acetylene-terminated oligomer. This entraps any residual solvent and air. As a result, the cured neat resin, composite, and adhesive joint contain voids and cracks which result in poor mechanical performance.
Another important factor contributing to the poor mechanical performance, particularly elevated temperature mechanical properties, is a lack of high degree of crosslinking. The crosslinking reaction occurs in stage two. The reacting group is the conjugated polyene marked in the rectangular area in the middle of the reaction scheme. Since this reaction site is buried in the center of a cluster, it is extremely difficult sterically for the polyene to interact with another molecule of the polyene. Consequently, a very high processing temperature is required to effect the crosslinking reaction.
The novelty of the present invention lies in the concept that if stage one of the reaction is slowed down and stage two is accelerated, a well-consolidated composite will result. The semi-IPN reaction system of the present invention is designed to exploit this concept.
An object of the present invention is to prepare a tough, processable semi-IPN from a thermosetting and a thermoplastic polyimide. The semi-IPN reaction system is so designed to undergo chain extension below 300.degree. C., whereby the flow and the reaction rate are decreased and the processing window is broadened and, upon heating above 300.degree. C., the flow is increased and crosslinking occurs at a rate which allows for the formation of a void-free polymer network.
Another object of the present invention is to form a simultaneous semi-interpenetrating network from a thermosetting polyimide and a thermoplastic monomer precursor solution.
Another object of the present invention is to improve the processing of the Thermid.RTM. materials.
Another object of the present invention is to improve the processing of NR150-B2.
Another object of the present invention is to prepare molding compounds, adhesives, and polymer matrix composites from the semi-interpenetrating network.